posixage: use caller context for lock waits

[ilopezluna]

• use the caller context to bound lock acquisition instead of the previous internal 100ms wait
• keep stale lock recovery internal with the existing fixed 30s age threshold
• update posixage flock tests and README locking docs for context-driven waits

Verification:

• go test ./store/posixage/...
• git diff --check

[Benehiko]

The capability is right, but the API shape needs rework before merging.

Drop WithLockTimeout and flock.Config.LockTimeout. context.Context already expresses "how long to wait" — callers who want a bounded wait pass context.WithTimeout; callers who want no timeout pass a ctx without a deadline. That gives you the disable-timeout case for free, no new option needed.

Keep WithStaleLockTimeout only. It's a file-age threshold, not a wait duration — semantically distinct, belongs as a store-level option.

Move stale recovery trigger off LockTimeout. With LockTimeout gone, trigger recovery on first-attempt failure instead (try once → on fail, attempt recovery if StaleLockTimeout > 0 and ctx not canceled → retry until ctx done).

Proposed signature:

func TryLock(ctx context.Context, root *os.Root, staleAfter time.Duration) (UnlockFunc, error)

Defaults stay (no deadline = wait forever; 30s stale). Drops one option, one struct, and the LockTimeout/StaleLockTimeout coupling.

store/posixage/internal/flock/flock.go:

• tryLock — drop cfg Config param, take staleAfter time.Duration; restructure to "first-attempt → recover → retry til ctx done"
• retryLock — drop timeout param, retry purely against ctx
• TryLock / TryRLock — new signatures (ctx, root, staleAfter)
• Delete Config, DefaultConfig, defaultLockTimeout const

store/posixage/store.go:

• fileStore.tryLock / tryRLock — pass f.staleLockTimeout instead of f.lockConfig
• config struct — replace lockConfig flock.Config with staleLockTimeout time.Duration
• Delete WithLockTimeout
• WithStaleLockTimeout — stays, writes to new field
• New — init default staleLockTimeout = 30 * time.Second (move const from flock pkg or expose getter)

store/posixage/internal/flock/flock_test.go:

• All tests passing Config{...} — rewrite to pass staleAfter directly; tests covering LockTimeout behavior become ctx-deadline
  tests

store/posixage/store_test.go:

• Any test exercising WithLockTimeout — remove or convert to ctx-deadline

store/posixage/README.md:

• Update the new lock-config section