Make Gmail thread replies first-class

[hargup]

Summary

• Gmail thread view becomes the default projection; thread replies are now first-class children with stable, namespaced remote IDs (gmail-thread:<mailbox>:<thread-id> / thread-child message IDs).
• Reply drafts can be created directly from a hydrated thread (loc create gmail-reply), carrying the Gmail thread ID and RFC reply headers.
• Thread directory and message/reply file names drop the leading date and switch to <title_slug>_<id> / <title_slug>_<id>.md (was <date>-<title_slug>-<id>), trading chronological sort order for stable, readable names.
• Merged current main in to pick up the cloud sandbox data plane work and resolve one conflict in crates/localityd/src/push.rs (kept both branches' additions: main's new local_projection_items_for_plan and this branch's rename of block_ambiguous_gmail_send_replay to retryable_gmail_draft_create_push_id).

Test plan

• cargo build --workspace --tests
• cargo test -p locality-gmail (90 passed)
• cargo test -p localityd -p loc-cli -p locality-core (all passed, including source_guidance_distinguishes_writable_and_read_only_sources, which was failing pre-merge due to this branch's stale templates/mount/AGENTS.md)
• docs/gmail-connector.md and docs/cli.md updated with the new naming examples

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