Make Treemon own worktree forking with a post-fork setup hook

[0101]

Problem

Creating a worktree from the dashboard used to delegate forking to a repo-local fork.ps1 / fork.sh, passing only the branch name. That path had real bugs:

• The fork script silently ignored the selected source branch.
• "New worktree from main" could fork from a stale local main in fork workflows where the upstream is ahead.
• Resolution logic ran for a base the script never actually used.

Changes

Treemon owns forking (upstream-aware base)

• Treemon now creates the worktree itself: git worktree add -b {name} {parentDir}/tm-{name} {baseRef}.
• Base resolution fetches the base from the upstream remote and prefers the remote-tracking ref {remote}/{base} over a possibly-stale local branch, falling back to the local {base} when no remote-tracking ref exists. No worktree needs the base checked out; fetch/remote failures fall back to whatever ref is available.

post-fork setup hook

• After creation, an optional post-fork.ps1 (Windows) / post-fork.sh (Unix) runs inside the new worktree, receiving {worktreePath} {sourceRepoRoot} {baseRef} {branchName}. It is for setup only (links, dependency install); a failure is a non-fatal warning since the worktree already exists.
• Treemon's own post-fork.ps1 wires .claude / data via directory junctions (no UAC elevation or Developer Mode required — important because the server spawns the hook) and runs bd init + npm install.
• Legacy fork.ps1 / fork.sh are no longer executed. If one is present, creation still succeeds but returns a warning to migrate setup into post-fork.*. Treemon's fork.ps1 is deleted and split into post-fork.ps1 (setup) and review-worktree.ps1 (the remote review-branch workflow, which reuses post-fork.ps1).

Warnings surfaced to UI + CLI

• createWorktree returns Result<CreateWorktreeWarnings, string> (a named string list alias). Warnings render in the create modal (new warning header/message styles) and on the CLI console.

Hardening

• Reject leading-dash branch names (regex aligned with TreemonConfig) and pass -- before the git fetch refspec to prevent git argument injection.
• ProcessRunner.runResultWithTimeout gives the post-fork hook a 10-minute budget so npm install / bd init are not killed by the 60s default used for quick git probes; the timeout warning now states dependencies may be incomplete.

Code quality

• CreateWorktreeModal.update flattened into a single (msg, modal) tuple match.
• legacyForkScriptWarning made pure — File.Exists hoisted to the asyncResult boundary.
• Shared git test helpers extracted into GitTestHelpers.fs; RemoveWorktreeTests and CreateWorktreeServerTests deduped.
• Spec (docs/spec/worktree-monitor.md) updated to describe the new create flow.

Tests

• New CreateWorktreeServerTests cover base-ref precedence and end-to-end create against real git repos: forks from the upstream tip over a stale local base, remote-only base, missing base (errors and creates nothing), legacy-script warning, and post-fork execution inside the new worktree.
• Build clean (0 warnings); 958 Fast tests + 83 targeted CreateWorktree/RemoveWorktree tests pass. Junction creation smoke-tested non-elevated.

[Copilot]

Pull request overview

This PR makes Treemon own git worktree forking instead of delegating to a repo-local fork.ps1/fork.sh. Treemon now fetches the base branch from the upstream remote and forks the new worktree directly via git worktree add -b {name} {parentDir}/tm-{name} {baseRef}, preferring the remote-tracking ref ({remote}/{base}) over a possibly-stale local branch. After creation it runs an optional, non-fatal post-fork.ps1/post-fork.sh setup hook inside the new worktree. Legacy fork scripts are no longer executed; their presence yields a migration warning. Warnings now flow through createWorktree (Result<CreateWorktreeWarnings, string>) to both the create modal and the CLI.

Changes:

• Server-side, upstream-aware forking with resolveBaseRef precedence, an optional post-fork hook (10-min timeout), and non-fatal warnings; branch-name regex tightened to reject leading dashes and -- added to git fetch refspecs.
• API contract change: createWorktree returns Result<CreateWorktreeWarnings, string>; modal/CLI updated, modal update flattened to a (msg, modal) tuple match with a new CreateWarning state and styling.
• fork.ps1 removed and split into post-fork.ps1 (junction-based setup) and review-worktree.ps1; shared git test helpers extracted to GitTestHelpers.fs; new CreateWorktreeServerTests; spec updated.

Show a summary per file

File	Description
src/Server/GitWorktree.fs	Core change: base-ref resolution, fetch hardening, post-fork hook, warning assembly
src/Server/WorktreeApi.fs	Wires createWorktree to new signature; returns warnings; fixture/read-only modes updated
src/Server/ProcessRunner.fs	Parameterizes timeout; adds runResultWithTimeout and shared toResult
src/Shared/Types.fs	Adds CreateWorktreeWarnings alias; updates createWorktree return type
src/Client/CreateWorktreeModal.fs	Flattens update to tuple match; adds CreateWarning state and view
src/Client/index.html	Adds warning header/message CSS classes
src/Cli/Program.fs	new command prints warnings to stderr via tryCallServer
post-fork.ps1	New junction-based setup hook (no elevation needed)
review-worktree.ps1	New manual remote-review-branch workflow reusing post-fork
fork.ps1	Deleted (legacy symlink/elevation flow)
src/Tests/GitTestHelpers.fs	Extracted shared git repo test helpers
src/Tests/CreateWorktreeServerTests.fs	New end-to-end + unit tests for base-ref precedence and creation
src/Tests/CreateWorktreeTests.fs	Updates for Ok [] and new warning case
src/Tests/RemoveWorktreeTests.fs	Dedupes git helpers into GitTestHelpers
src/Tests/Tests.fsproj	Registers new test/helper files
docs/spec/worktree-monitor.md	Documents the new create flow

Copilot's findings

• Files reviewed: 16/16 changed files
• Comments generated: 2

[0101]

Good catch. In 82c8194 the test now writes post-fork.ps1 on Windows and post-fork.sh (invoked via bash) elsewhere, so it runs on ubuntu-latest instead of being Assert.Ignored. Also added a companion test for the post-fork failure-to-warning path.