Improve issue mine's no-team error and suggest linear config in repos

[schpetbot]

Follow-up to #253 / #254, addressing the other rough edge in the out-of-the-box experience:

❯ linear issue mine
✗ Failed to list issues: Could not determine team key from directory name or team flag

What changed

issue mine (and its list/l aliases) now reports the same error as issue query when no team can be determined, with an actionable suggestion — and when run inside a git work tree, it additionally points at linear config, which links the repository to a team by generating .linear.toml:

# outside a repo
✗ Failed to list issues: No default team configured and no team scope provided
  Use --team <key> to specify a team.

# inside a repo
✗ Failed to list issues: No default team configured and no team scope provided
  Use --team <key> to specify a team, or run `linear config` to link this repository to a team.

The old message was also inaccurate: the team never comes from the directory name — only from --team, LINEAR_TEAM_ID, or team_id config.

Repo detection is a new isInsideGitRepo() helper in src/utils/git.ts that is deliberately best-effort: any failure (git missing, not a repo, dubious ownership, spawn error) yields false, so the optional hint can never replace the actionable team error with a git crash. It checks git rev-parse --is-inside-work-tree's stdout, not just its exit code (which is 0-with-"false" inside .git dirs and bare repos), and covers colocated jj repos via their .git.

Tests

• Unit tests for isInsideGitRepo (non-repo, repo, nested dir).
• Two subprocess regression tests asserting the exact two-line stderr for the in-repo and non-repo variants. Subprocess with a clean env/cwd is required: the repo's own .linear.toml sets team_id, so the error path is unreachable in-process from the repo root.

Deferred

The same stale message exists in cycle list, cycle view, team states, and team members; they can adopt isInsideGitRepo() and this error shape in a follow-up. issue start's "Could not determine team ID" and adding the linear config hint to issue query are likewise out of scope here.