fix gmail packaging gaps for binary installs (PNX-171)

[cjus]

Summary

Spun out of PNX-168 (Bun packaging). Two gaps surfaced verifying v0.3.0-rc1 on a clean machine:

• Hardcoded ~/.solrac/gmail/ path ignored SOLRAC_HOME, so operators with a non-default home couldn't use gmail at all.
• OAuth bootstrap was source-tree-only (scripts/gmail-auth.ts). The curl-pipe binary install ships solrac only — no bun, no scripts — leaving binary-install operators with no way to mint per-account token files.

Why

The integration runtime worked fine in rc1 (deps bundled, self-gates correctly) — only the bootstrap UX and path resolution were broken. This blocks gmail in any non-default deployment and on every curl-pipe install.

Changes

• ctx.solracHome on IntegrationContext — blessed integrations now resolve paths under $SOLRAC_HOME/integrations/<name>/. Forward-compatible namespace for future blessed integrations (Slack, Linear, etc.).
• createGmailClientApi(solracHome) factory in src/integrations-builtin/gmail/client.ts replaces module-level path constants. State is per-instance, not module-global.
• solrac gmail-auth <alias> subcommand (new src/integrations-builtin/gmail/auth-cli.ts) — ships in the compiled binary. Banner prints resolved solracHome + gmailDir. Missing-credentials error links to the specific Google Cloud Console URLs.
• Pre-loadConfig dispatch in main.ts so subcommands run before env validation — fresh installs can authenticate gmail before configuring the bot.
• Deleted scripts/gmail-auth.ts — single path through the subcommand.
• Docs updated: USAGE.md (new flow + pre-PNX-171 migration note), INSTALL.md (CLI subcommands table).

Impact

• Path layout change: state moves from ~/.solrac/gmail/ to $SOLRAC_HOME/integrations/gmail/. Migration is doc-only (one mv command, no auto-migrate). rc1 is rc — adoption is low.
• createIntegrationContext() signature change: now takes solracHome: string. Updated all callers (smoke harnesses, tests).
• scripts/gmail-auth.ts gone: any operator docs/runbooks referencing it need to switch to solrac gmail-auth <alias>.

Test plan

• npm run typecheck — clean
• bun test — 722 pass / 0 fail
• bun test src/integrations-builtin/gmail/client.test.ts — 8 new path-resolution tests pass
• Manual: bun src/main.ts gmail-auth — usage error
• Manual: bun src/main.ts gmail-auth "bad alias!" — validation error
• Manual: SOLRAC_HOME=/tmp/solrac-test bun src/main.ts gmail-auth ieee — banner + GCP URL hint on missing credentials
• On a binary install: verify solrac gmail-auth <alias> opens browser and writes tokens to $SOLRAC_HOME/integrations/gmail/
• Pre-existing operators: verify mv ~/.solrac/gmail $SOLRAC_HOME/integrations/gmail followed by restart loads accounts as before