feat(worktree): add worktree create verb (AB#3096, PR 1b3)

[PolyphonyRequiem]

Summary

Adds the polyphony worktree create --apex N --branch B [--ref R] verb — the per-item worktree-creation primitive used by every workflow step that needs an impl/, plan/, mg/, or evidence/ worktree under an apex run. Mirrors the structure of worktree init-apex (PR #311 / PR 1b2) with branch-grammar validation, bootstrap-dependency checks, and explicit --ref handling.

Closes AB#3096. Part of AB#3085 (bare-repo + per-run worktree epic).

Stacking

Stacked on PR #311 (PR 1b2: worktree init-apex). Branch feature/worktree-create is off feature/worktree-init-apex tip. The diff against main will be clean once #311 merges; please merge #311 first.

What this PR does

Implements:

polyphony worktree create --apex {N} --branch {B} [--ref {R}]

Behavior:

1. Validates --apex > 0 and that --branch parses against the canonical grammar (from BranchSlug.TryParse: feature/{r}, plan/{r}[-{item_id}], mg/{r}_{mg_path}, impl/{r}-{item_id}, evidence/{r}-{item_id}).
2. Refuses feature/{N} branches with unsupported_branch_kind — init-apex is the canonical path for the apex feature worktree.
3. Asserts parsed.RootId == --apex (else branch_apex_mismatch).
4. Resolves runs_root and main_path via RunsRootResolver (from PR 1b1).
5. Asserts the derived target path {runs_root}/apex-{N}/{slug}/ lives under runs_root AND not under main_path via PathBoundary (from PR 1b2). The hijack is structurally impossible.
6. Asserts the apex was bootstrapped: {runs_root}/apex-{N}/feature-{N} must already be a registered worktree on feature/{N}. Otherwise apex_not_initialized (catches partial-init, missing apex root, manual mkdir).
7. Applies a deterministic create-or-attach matrix (path checks first, then branch state).

Reason taxonomy

reason	When
invalid_apex	apex <= 0
invalid_branch	grammar-rejected by BranchSlug.TryParse
branch_apex_mismatch	parsed root id differs from --apex
unsupported_branch_kind	branch is feature/{N} (use init-apex)
common_dir_unavailable	git rev-parse --git-common-dir failed
filesystem_failure	resolved path violated a boundary invariant
apex_not_initialized	{apex_root}/feature-{N} is not a registered worktree on feature/{N}
git_failure	git non-zero with no race recovery, or porcelain parse threw
path_exists_wrong_branch	target path is a worktree on a different branch
path_exists_not_worktree	target path exists (file or dir) but is not registered
branch_in_use	local branch already checked out at another worktree
remote_branch_exists	local branch missing AND origin/{branch} exists (refused regardless of --ref)
ref_required	local branch missing AND --ref empty (after remote check passed)

outcome is one of: created, attached, idempotent, failed.

Rubber-duck-validated decisions

• Bootstrap dependency is structural (not just Directory.Exists). The check that {apex_root}/feature-{N} is a worktree on feature/{N} is derived from the same git worktree list --porcelain call we already make for path-existence — no extra git invocations. Catches partial-init, manual mkdir, and missing apex root in one signal.
• Reject feature/{N} branches. BranchSlug.TryParse accepts the Feature kind (and other verbs need that), but create rejects it with unsupported_branch_kind. Otherwise create would become an unprotected back-door around init-apex's remote-branch + always-root-from-main safety.
• remote_branch_exists always wins, even with --ref. Workflow-rerun safety: if the local branch is missing but origin/{branch} exists, refuse regardless of what was passed for --ref. The operator must explicitly fetch + branch first to avoid silent fork.
• No cwd-refusal. Path is self-derived from runs_root; cwd has no impact on safety. init-apex legitimately runs from main during bootstrap, and the same logic applies to create.
• No ref_unresolvable pre-check. Git is the authoritative resolver; a bad ref naturally surfaces as git_failure with stderr.
• Ref serializes as ref (not ref_spec). ref is valid JSON and doesn't collide with JSON Schema's $ref.
• Slug derivation: only the leading kind/ separator is replaced. impl/3085-3072 → impl-3085-3072; mg/3085_pg-foo → mg-3085_pg-foo (underscores preserved).

Files

• src/Polyphony/Models/WorktreeCreateResult.cs (new) — result record. Doc-comment is the authoritative reason taxonomy.
• src/Polyphony/Commands/WorktreeCommands.Create.cs (new) — partial verb. Reuses static helpers from WorktreeCommands.InitApex.cs (FindByPath, NormalizePath, PathExists, GitStderrOrFallback, ProbeIdempotentAsync, ParsePorcelain).
• src/Polyphony/PolyphonyJsonContext.cs — [JsonSerializable(typeof(WorktreeCreateResult))].
• src/Polyphony/Commands/WorktreeCommands.cs — <see cref="Create"/> doc-line.
• tests/Polyphony.Tests/Commands/WorktreeCommandsCreateTests.cs (new) — 27 tests covering arg validation, branch grammar (incl. feature/N refusal), partial-init detection, path-exists matrix, branch-state matrix (incl. ref_required, remote-refusal-with-ref, attach-without-ref), race tolerance, JSON contract.
• artifacts/verb-output-schemas.json — regenerated; adds worktree create entry.

What this PR does NOT do

• Does not invoke worktree create from any workflow YAML or PowerShell helper. That's PR 4 (workflow integration + driver pre-flight).
• Does not add worktree gc. That's PR 7.
• Does not change launcher behavior. That's PR 3.
• Does not change branch invariants documented in polyphony-branch-model.

Validation

• 27/27 new tests pass in isolation
• Full Polyphony.Tests suite green (3315 passed; one documented parallel-race flake passed on retry)
• All 4 CI lints pass locally: prose-children, jinja-resolver, version-drift, conductor-validate
• Schema artifact regenerated; worktree create entry contains apex/branch/ref inputs and Polyphony.WorktreeCreateResult result type