refactor(core): rename the execution claim column

[kitlangton]

Why

session_v2.time_suspended holds the write-ahead execution claim, including while a Session is actively running. The suspension name misrepresents the recovery marker and makes claim queries harder to read.

What Changes

Rename the column to execution_claimed_at and its partial index to session_v2_execution_claimed_at_idx. Claim acquisition, release, and restart selection keep their existing behavior.

Before migration	After migration
Claimed root: time_suspended = 1234, resume_attempts = 2	execution_claimed_at = 1234, resume_attempts = 2
Claimed child: time_suspended = 5678, resume_attempts = 1	execution_claimed_at = 5678, resume_attempts = 1
Unclaimed Session: time_suspended = NULL	execution_claimed_at = NULL
Fresh database	Nullable execution_claimed_at; resume_attempts still defaults to zero

The new migration uses SQLite's column rename, not a table rebuild. It preserves activity timestamps and dependent history, and recreates the partial index under the new name. The repository's migration generator updates the registry, current-schema bootstrap, and Drizzle snapshot; historical migrations are unchanged. The previous-V2 importer still reads the legacy source column and writes the renamed destination.

Scope

Storage naming and regression coverage only; no public Protocol/API changes. Subagent recovery refactoring and event-based child-claim release remain separate work.

Verification

Run with Bun 1.4.0:

cd packages/core
bun run migration --name execution_claimed_at
# Selected time_suspended -> execution_claimed_at in Drizzle's rename prompt.
bun run migration --check
bun run test test/execution-claim-migration.test.ts test/database-migration.test.ts test/v1-migration.test.ts test/session-create.test.ts test/session-projector.test.ts test/session-execution.test.ts
bun run test
bun typecheck
cd ../..
bunx prettier --check $(git diff --name-only c992716523...HEAD)
git diff --check c992716523...HEAD
git push -u origin claim-column

• Focused tests: 136 passed, 0 failed. Coverage includes fresh bootstrap, registry-driven upgrade/reapplication, preserved claims and retry counts, dependent history/foreign keys, and previous-V2 import.
• Full core suite: 4,019 passed, 39 skipped, 0 failed across 228 files (89.21 seconds).
• Core typecheck, migration freshness check, formatting, and whitespace checks passed.
• Push ran the unmodified pre-push hook: 33 typecheck tasks successful, including 27 cache hits.
• Dependencies installed with bun install --frozen-lockfile; no lockfile change.