feat: add SessionStart hook for early auth/scope and project-existence warning

[gringolito]

Closes #39

Summary

• Adds scripts/session-start-check.sh — runs at session start in any repo with .claude/backlog-project.json; silently exits if the file is absent (no noise in unrelated repos)
• Registers it in hooks/hooks.json as a SessionStart hook via ${CLAUDE_PLUGIN_ROOT}/scripts/session-start-check.sh
• Always exits 0 — warns but never blocks the session

What the hook checks

Check 1 — required token scopes: parses gh auth status --hostname github.com and verifies repo, project, read:user are present (plus read:org when the project owner is a GitHub organisation). On failure, emits a banner naming the missing scopes and the exact gh auth refresh --scopes … command.

Check 2 — project still exists: queries the live project ID via gh project view and compares it against the stored project_id from .claude/backlog-project.json. On mismatch or missing project, emits a banner with the remediation (/initialize-backlog or delete the metadata file).

Acceptance Criteria

AC	Status
scripts/session-start-check.sh exists, executable, #!/usr/bin/env bash shebang	✅
hooks/hooks.json registers a SessionStart hook invoking the script	✅
Silent when no .claude/backlog-project.json present	✅ tested
Silent when token fully scoped + project exists	✅ (happy path verified)
Missing scope → banner + exact gh auth refresh … command	✅ tested
Wrong project_id → banner + remediation message	✅ (code path verified)
Both failure cases exit 0	✅ tested
Banner appears at session start, before any prompt	✅ (SessionStart fires before user input)

Note: The scratch-repo smoke test (claude --debug shows hook registered) and the missing-project-id path require a live install and are not automatable in CI — verify manually before merging.

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