fix(context-pivot): restore visibility on session start

[outlier27-cell]

Problem

context-pivot ignored the ExtensionContext supplied at session_start, so a resumed or reloaded session already above the 30k threshold hid the tool until the next agent turn.

Closes #744

Value

Context-pivot availability now immediately reflects the active Session's known context usage.

Approach

• Pass the existing session-start context through the existing visibility helper.
• Extend the threshold lifecycle test to cover a 30k-token session start.

Validation

• node --test --experimental-strip-types tests/extensions/context-pivot/index.test.ts
• bun run check
• Exact-head GitHub CI is tracked for the final cross-platform result.

Impact

No tool semantics or threshold changes. This only removes the post-resume visibility delay.

[outlier27-cell]

@tt-a1i 请审核此 PR。当前精确提交的 GitHub CI 已全部通过，修改范围与回归测试均已在 PR 说明中列明；现仅等待维护者审核。