Add --project-id/--org-id/--branch-id to neon init

[jeff-at-neon]

Summary

• Adds three optional flags to neon init — --project-id, --org-id, --branch-id — that streamline setup for users who already have a Neon project, in both agent and interactive modes.
• When --project-id is provided, the getting-started flow collapses its org/project select-or-create-or-context chain into a single verified neon link step (verifies the IDs against the account, infers the org, records the branch, and writes .neon). --org-id alone scopes project selection to that org and skips org selection; --branch-id is recorded in .neon so neon env pull targets that branch.
• IDs are validated at the CLI boundary (new shared assertSafeId in src/init/ids.ts) before any shell interpolation, so a malformed ID is rejected early in the caller's expected shape (JSON in agent mode).
• The --data '{"step":"db"}' agent path also gains branchId, threaded into the connection-string command and project_ready payload.

Behavior

Provided	Effect
--project-id	Skips org + project selection; one verified link, then env pull
--org-id alone	Skips org selection; still lists/creates a project in that org
--branch-id	Recorded in .neon so env pull targets that branch

The no-ID path is byte-identical to before (only the --help snapshot changed, to list the new flags).

Test plan

• pnpm typecheck (tsc --noEmit)
• biome check (error-on-warnings) clean on changed files
• vitest run init suites — 310/310 pass (agent_snapshot, init, route_command, orchestrate)
• Regenerated --help snapshot (adds the three flags only)
• Smoke test on built binary: flags appear in --help; malformed --project-id rejected as JSON in agent mode
• Consider whether a changeset is needed for the neon package release

This pull request and its description were written by Isaac.

[jeff-at-neon]

Superseded by #428, which makes neon init minimal (install tooling, then hand off to the agent). The --project-id/--org-id/--branch-id flags this PR added are removed there since init no longer links projects.