donate: source agent from skill checkout so donate works from a non-git cwd

[boweiliu]

Problem

mngr donate raised Not inside a git repository. Either run from within a git repo, or specify --from to set the source explicitly. when run from a non-git directory — the common case for users who install mngr from a wheel/PR rather than a monorepo checkout (e.g. someone who installed from the mngr/donate-auth-fix branch via uv tool install).

Root cause

build_create_argv built the mngr create invocation with no --from. With no --from, mngr create falls back to sourcing the agent from the current directory (_resolve_source_location -> find_git_worktree_root(cwd)), which returns None outside a git repo and raises UserInputError. The donate README even stated the assumption ("Run it from a trusted git repo, since the created agent is sourced from the current directory") — that assumption is the bug for installed-from-source users.

Fix

build_create_argv now passes --from <skill_dir>. The skill checkout (prepare_skill_dir clones the skill repo into ~/.mngr/donate-skills/<skill>/) is always a git repo, so the agent sources from it regardless of the caller's cwd. The donation agent only talks to the skill's coordination server (its own message says so), so the skill checkout is the correct source anyway.

_clear_stale_worktree now runs git -C <skill_dir> (where the agent worktree is now rooted) instead of git in the caller's cwd, so stale-worktree cleanup still works when the caller's cwd isn't a git repo.

Verification

• Unit tests: 73 passed (libs/mngr_donate).
• Repro: a script exercising the real build_create_argv from a non-git cwd — pre-fix hit "Not inside a git repository", post-fix passes source resolution and reaches the auth stage.
• E2e: ran a full mngr donate tick from /tmp/repro_not_a_git (a non-git dir) with a real token — exit 0, the agent launched, leased 10 papers, and reviewed them. Confirmed the agent worktree was rooted in the skill cache and that _clear_stale_worktree cleans up the leftover mngr/<agent> branch against the skill dir.

Stacking

Stacked on top of #2460 (mngr/donate-auth-fix, the --pass-env auth fix), which is itself stacked on #2417 (the base mngr_donate plugin). Merge order: 2417 -> 2460 -> this PR.