feat(sync): add prompt reference workspaces

[ctawiah]

Context

Prompt sync needs a stable repository root and a small adapter boundary for prompt content maintained outside .launchdarkly. This layer adds those boundaries without creating local sync state or exposing a command.

What changes

• Requires an initialized Git repository and resolves one canonical workspace root.
• Defines the adapter contract used to parse and render external prompt formats.
• Adds plain-markdown as the first adapter.
• Resolves referenced files safely within the repository.

Review focus

• Is workspace discovery predictable from nested directories?
• Does the adapter contract leave room for additional prompt formats?
• Are referenced paths constrained to the repository?

Verification

• go test ./internal/sync/...
• git diff --check

Related changes

Review the stack in this order:

1. Sync foundations
2. Prompt reference workspaces
3. Prompt sync manifests
4. Local prompt workspaces
5. Bootstrap prompt workspaces
6. Link and detach prompt resources
7. Prompt reconciliation
8. Prompt sync orchestration
9. Prompt sync command

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Note

Overview
Introduces the prompt reference foundation for sync: a pluggable adapter contract (Parse/Render) with shared Prompt types, plus plain-markdown as the first format (whole file → one system message, line-ending normalization, round-trip via reference.Parse / ApplyToVariation / Render).

Adds Git-backed workspace discovery: repository.FindGitRepository (git rev-parse --show-toplevel, LC_ALL=C for stable “not a git repository” detection) and source.Resolver, which returns a canonical repo root (absolute path, symlinks resolved) or ErrGitRequired when not inside an initialized repo.

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[nieblara]

WOO!

Non-blocking: findGitRepository reports every git failure as "not found." So if you're inside a repo but git refuses to run, say because of "detected dubious ownership" (common in containers, WSL, and CI) or a permissions error, you get told to "initialize a Git repository." That might be a bit confusing to users, and it could send an agent using these tools down the wrong path too.

One option is to split this into three cases: git isn't installed, the directory isn't a repo, and some other git error. For the last one, include the stderr from exec.ExitError. That way the error return actually gets used. Up to you!

[ctawiah]

Addressed this feedback. Repository discovery now distinguishes a missing Git executable, an explicit “not a git repository” response, and other Git failures. Operational failures preserve Git’s stderr, with coverage for dubious ownership and errors that do not include stderr.

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

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