feat(telemetry): emit session_end for exec and surface on cli_start

[TheGreatAxios]

Summary

• Exec runner emits one best-effort session_end per run from the settle path, mirroring the TUI exit reporter's status/exit_reason contract.
• Pre-try setup failures (session dir/state/persist) and unconfigured exec each emit a failed session_end, so every cli_start with surface exec is paired with exactly one session_end.
• Missing-prompt usage errors deliberately emit nothing: exit 2 with no run started is not a session, and counting it as failed would pollute failed counts.
• cli_start allowlist passes surface; startup tags exec runs exec and interactive runs tui; held first-run activation defaults to tui with an explicit override.

Verification

• bun run lint passes (oxfmt + oxlint, 0 warnings/errors)
• bun run typecheck passes
• bun run build passes (via pre-commit hook)
• bun run check:projects-dir-guard passes
• New tests/unit/telemetry-session-end-exec.test.ts: 12 pass, covering the pre-try setup failure, unconfigured-exec, and success-path exactly-one cases plus the missing-prompt no-emit case
• Related suites pass: telemetry files 78 pass, tests/unit/exec/ 35 pass, tests/unit/index.test.ts 6 pass
• Full bun run test: 8597 pass, 2 fail — both environmental and unrelated (transient ENOSPC on a 99%-full disk for an onboarding write test, since re-passing in isolation; missing ts-prune binary for the dead-exports guard end-to-end test, failing identically on clean main)

Fixes CL-9020

[linear-code]

CL-9020