docs: merge the stacked doc comments the previous commits left behind

Cosmetic, but the kind that quietly costs: JSDoc tooling attaches only the
nearest block, so a stacked second block silently hides the first.

- write() had two: the original description with @param and @example, then a
  @returns-only block added on top, which dropped the params and examples from
  hover. Merged into one. The @returns wording is also honest now — write() still
  discards the data without a PTY; what changed is that it says so.
- forgetInputSeq had been inserted BETWEEN shouldApplyInput's detailed doc comment
  and its declaration, leaving that function undocumented on hover and the doc
  attached to the wrong thing. Moved below.
- The mock kept an orphaned one-line comment above failWrites' own block.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/mocks/mock-session.ts

@@ -30,7 +30,6 @@ export class MockSession extends EventEmitter {
     this._muxName = `codeman-test-${id.slice(0, 8)}`;
   }
 
-  /** Direct PTY write (used by session.write()) */
   /**
    * Set to simulate a session whose PTY is gone: both write paths report failure,
    * which is the state in which input used to disappear silently.