feat(skills): a files arm — progressive disclosure through Read, not a deferred tool

The `index` arm retrieves through `ReadMcpResourceTool`, which is DEFERRED:
absent from the agent's default tool list until `ToolSearch` loads it. Across
three matched production runs (same recipe, same task, same three offered
uris) it retrieved 1 skill in 9 chances:

    01a07812  delegation forced      no instruction    0/3
    01a0842e  no delegation          no instruction    1/3
    01a09877  no delegation          told to load it   0/3

The third run is the decisive one. The preamble said in plain words to run
ToolSearch first; all three prompts carried it; zero ToolSearch calls, and the
three reasoning narratives never mention skills at all. The section was not
declined, it was never engaged with. Instruction is not the lever.

`Read` is a core tool. Never deferred, and every one of those agents used it.
So this arm keeps progressive disclosure exactly as `index` has it — a name, a
`when_to_use`, and a pointer the agent has to follow — and changes only what
the pointer is: a path under /mission/skills instead of an MCP uri. The bodies
are written into the container at launch (every visible skill, one tar upload;
bindings resolve per agent at turn time so a per-mission subset is not knowable
here) and a `Read` of that path is a tapped tool call, so Trigger is exactly as
observable as before.

A third arm and not a replacement, selected per mission like the others, so
the comparison runs against one binary. `resolve` falls back to `inline` when
the files were not written, for the reason `index` does: a pointer to nothing
reads as an agent ignoring its skills.

The writer and reader of a path are one pair of functions
(`skill_file_path` / `skill_from_file_path`), matched by the scorer through
the same seam `parse_uri` uses, and the end-to-end test fails when the matcher
is broken. `Mode::is_retrieval` exists so the next arm cannot silently inherit
`inline`'s "not observable" for what is a miss.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WZb5A2kfVfjpdwSochkuHz

crates/cm-api/src/container_tool_hooks.rs

@@ -39,7 +39,7 @@ pub const SETTINGS_PATH: &str = "/root/toolhooks/settings.json";
 /// Where the `PostToolUse` tap appends, inside the container.
 pub const TAP_DIR: &str = "/root/toolhooks/tap";
 
-const INSTALL_TIMEOUT: Duration = Duration::from_secs(30);
+pub const INSTALL_TIMEOUT: Duration = Duration::from_secs(30);
 
 /// Install the pre-execution gate and the tool tap into a mission container.
 ///