Problem

src/gateway/sessions-resolve.ts currently rejects the legacy session alias agent:main:main. There is a deferred design question: should it remap agent:main:main onto the live default agent instead of rejecting it? This needs a reviewed boundary (when/whether the remap applies) before implementing — an unconsidered remap risks unintended cross-agent resolution.

Current state

• agent:main:main (legacy alias form) is rejected by sessions-resolve.ts.
• src/gateway/sessions-resolve-store.test.ts (~line 17) has a test.skip pending this remap-boundary design.
• The skip site carries an in-code TODO with the un-skip condition.

What to do

1. Design the remap boundary — define precisely under what conditions agent:main:main should resolve to the live default agent vs. stay rejected. Consider:
   • Backward-compatibility with legacy clients emitting agent:main:main.
   • Ambiguity / collision risk (must not let the alias resolve to an unintended agent).
   • Security: no cross-agent resolution or privilege leakage via the alias.
2. Implement the reviewed boundary in sessions-resolve.ts.
3. Un-skip src/gateway/sessions-resolve-store.test.ts (~line 17) and make it pass.

Acceptance criteria

• The remap-boundary decision is documented (in-code rationale and/or an ADR-style note).
• sessions-resolve.ts implements the reviewed boundary.
• The test.skip at sessions-resolve-store.test.ts (~line 17) is un-skipped and passes.
• No regression to existing session-resolution behavior for non-legacy aliases.

Context

Split out from the gateway-suite remediation tracker #2724 (itself from #2720, "enable the gateway behavioral test suite in CI") as focused design+implementation work. This issue is self-contained; no external context is required.