feat(gmail): export filters as WebUI XML

[steipete]

Summary

• make gog gmail filters export emit Gmail WebUI-importable Atom XML by default
• resolve Gmail label IDs to names and map common filter actions to WebUI XML properties
• keep the old API JSON export via --format json and global --json stdout compatibility
• update README, generated command docs, and changelog

Closes #174.

Testing

• go test -vet=off ./internal/cmd -run TestGmailFilters
• make build
• make docs-commands
• make docs-site
• live clawdbot@gmail.com smoke: created disposable label/filter, exported mailFilters.xml, parsed XML for hasTheWord, label, and shouldArchive, then deleted the filter and label
• live JSON compatibility smoke: ./bin/gog --account clawdbot@gmail.com gmail filters export --format json --no-input | jq 'has("filters")'\n- make ci

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve JSON transforms for file exports

When --format json --out ... is used, this branch marshals with json.MarshalIndent instead of outfmt.WriteJSON, so JSON-mode transforms (--results-only / --select) are no longer applied to the exported file. Before this commit, gog gmail filters export --out ... always wrote through outfmt.WriteJSON, so scripts relying on projected/unwrapped output now get a different file payload even though the same global JSON flags are set.

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