Cache repo name with sync.Once to avoid extra gh subprocess on every fetch

[joshuacrass]

Problem:
Every call to FetchIssues and FetchIssue in internal/github/issues.go spawns a gh repo view --json nameWithOwner subprocess via repoOwnerName() → FetchRepo(). At startup, with 3 parallel fetches (dashboard, issues, PRs), this fires 3–5 extra gh subprocesses beyond the actual data fetches. The repo name does not change during a hubcap session.

Solution:
Cache the result of repoOwnerName() / FetchRepo() after the first call using a sync.Once and package-level variables in the internal/github package. All subsequent calls return the cached value with no subprocess.

Acceptance Criteria:

• gh repo view is called at most once per hubcap session
• The cached value is used for all subsequent FetchIssues, FetchIssue, FetchPRs, and FetchPR calls
• Concurrent callers are safe (no data race on the cached value)
• go test -race ./... passes