Addresses three nits from the PR #457 review:

1. Drop the unused CLAUDE_/GEMINI_/CODEX_MEMORY_NAMESPACE_PREFIX exports
   from `constants.py`. Promoting them while `cli/ingest_cmd.py` still
   hardcodes the literals would have created a *new* parallel-literal
   seam — exactly what `feedback_drift_close_must_derive` warns against.
   Wiring the ingest CLI is a separate change, not this PR's scope.

2. Drop the `_AGENT_NAMESPACE_PREFIX = AGENT_NAMESPACE_PREFIX` re-export
   alias in `multi_agent.py`. The "Re-export for callers that previously
   imported the local-private symbol" comment was misleading —
   underscore-private symbols have no external callers by definition,
   and grep confirms zero in-repo references on this branch. Stacked
   PRs (#459 / #461) that import the underscore-private symbol get
   updated when they rebase onto this PR; the public
   `AGENT_NAMESPACE_PREFIX` is the right import target.

3. Update the comment above `config.search.system_namespace_prefixes =
   ["archive:"]` in `test_trust_ux.py:68`. After this PR's default flip
   the override is *narrowing* (default has both `archive:` and
   `agent-runtime:`), not "spelling out" — the new comment says so and
   notes that multi-agent isolation has its own coverage in
   `test_multi_agent`.

No behavior change. 2335 tests still pass.

Co-Authored-By: Claude <[email protected]>

* fix(multi-agent): hide agent-runtime: from default mem_search

`search.system_namespace_prefixes` default was `["archive:"]` only, so
agent A's private chunks (created by `mem_agent_register` /
`mem_agent_search`) leaked into agent B's `mem_search(namespace=None)`
results — directly contradicting the namespace-based isolation
guarantee documented on the multi-agent guide.

Extends the default to `["archive:", "agent-runtime:"]` and consolidates
the literal in a new `memtomem.constants` module so config, MCP tools,
and CLI all derive the prefix from one source. `mem_agent_search` is
unaffected because it builds an explicit namespace filter; the hidden
count surfaces through the existing `hidden_system_ns` hint.

Behavior change: callers relying on plain `mem_search` to read
`agent-runtime:*` chunks must now either pin `namespace=` or override
`search.system_namespace_prefixes: []` in `config.json`. Documented
under [Unreleased] / Changed.

Threat model: `agent-runtime:<id>` is a *convenience* isolation
boundary, not a security boundary. Direct storage access (raw
`namespace=`, ingest `--namespace`, list_namespaces) still reaches
private chunks; this PR closes the default-search UX leak only.

Test surface (test_multi_agent.py):
- `TestDefaultIsolation` pins constants, default factory freshness,
  and `Mem2MemConfig` default — derives from the constant per
  `feedback_pin_test_constant_over_source_scan`.
- `TestAgentRuntimeIsolationPipeline` end-to-end: `agent-runtime:planner`
  chunks excluded from `namespace=None` search but reachable via
  pinned `namespace=`; `shared` namespace stays surfaceable.

Co-Authored-By: Claude <[email protected]>

* review fixup: drop dead alias/symbols + correct trust_ux comment

Addresses three nits from the PR #457 review:

1. Drop the unused CLAUDE_/GEMINI_/CODEX_MEMORY_NAMESPACE_PREFIX exports
   from `constants.py`. Promoting them while `cli/ingest_cmd.py` still
   hardcodes the literals would have created a *new* parallel-literal
   seam — exactly what `feedback_drift_close_must_derive` warns against.
   Wiring the ingest CLI is a separate change, not this PR's scope.

2. Drop the `_AGENT_NAMESPACE_PREFIX = AGENT_NAMESPACE_PREFIX` re-export
   alias in `multi_agent.py`. The "Re-export for callers that previously
   imported the local-private symbol" comment was misleading —
   underscore-private symbols have no external callers by definition,
   and grep confirms zero in-repo references on this branch. Stacked
   PRs (#459 / #461) that import the underscore-private symbol get
   updated when they rebase onto this PR; the public
   `AGENT_NAMESPACE_PREFIX` is the right import target.

3. Update the comment above `config.search.system_namespace_prefixes =
   ["archive:"]` in `test_trust_ux.py:68`. After this PR's default flip
   the override is *narrowing* (default has both `archive:` and
   `agent-runtime:`), not "spelling out" — the new comment says so and
   notes that multi-agent isolation has its own coverage in
   `test_multi_agent`.

No behavior change. 2335 tests still pass.

Co-Authored-By: Claude <[email protected]>

---------

Co-authored-by: pandas-studio <[email protected]>
Co-authored-by: Claude <[email protected]>