feat: add retry logic, concurrent fetching, and extended commands

Resilience:
- RetryTransport with circuit breaker for 429/5xx resilience
- Exponential backoff with jitter, respects Retry-After headers
- Circuit breaker auto-resets after 30s of successful requests

Performance:
- Concurrent gmail thread fetching (fixes N+1 query pattern)
- Bounded concurrency with semaphore (max 10 parallel)

New calendar commands:
- colors: list available event/calendar colors
- conflicts: check availability across calendars
- search: find events by text query
- time: show current time in multiple timezones

New gmail commands:
- autoforward: get/enable/disable auto-forwarding
- delegates: list/add/remove mail delegation
- filters: list/create/delete inbox filters
- forwarding: manage forwarding addresses
- sendas: manage send-as aliases
- vacation: get/enable/disable vacation responder
- batch: bulk operations (mark-read, archive, label, delete)
- watch: Pub/Sub push with webhook forwarding

New services:
- Sheets: read/write/append spreadsheet data
- Tasks: manage tasklists and tasks

Developer experience:
- Shell completion (bash, zsh, fish, powershell)
- version command with build info
- --debug flag for verbose logging
- lefthook for pre-commit hooks

Documentation:
- Expanded README with examples
- Gmail watch/Pub/Sub guide (docs/watch.md)
- Architecture spec (docs/spec.md)
- Release process (docs/RELEASING.md)

Author: salmonumbrella

.lefthook.yml

@@ -0,0 +1,18 @@
+pre-commit:
+  parallel: true
+  commands:
+    fmt-check:
+      glob: "*.go"
+      run: gofmt -l . | grep . && echo "Run 'gofmt -w .' to fix" && exit 1 || exit 0
+    lint:
+      glob: "*.go"
+      run: golangci-lint run --timeout 2m
+    build:
+      glob: "*.go"
+      run: go build ./...
+
+pre-push:
+  parallel: true
+  commands:
+    test:
+      run: go test ./...