Remove Dead DM-History Helper Functions in `agent.c`

Background

While investigating how Didactyl maintains continuity across a DM "chat session", two functions in src/agent.c were found to be compiled-but-unreferenced, marked with __attribute__((unused)) to suppress compiler warnings:

append_recent_admin_dm_history
build_recent_admin_dm_history_messages (wraps the above)

These were part of an older design where the agent automatically injected the last N admin DM turns into every LLM call. The current architecture is skill-driven: DM history only reaches the LLM when an adopted, triggered skill explicitly pulls it in via the {{dm_history}} / {{nostr_dm_history(...)}} template variables or the nostr_dm_history tool (see tool_agent.c and tools_schema.c). The dead functions are now a source of confusion for anyone reading the code to understand chat-session behavior.

Goal

Remove the dead functions and any symbols used exclusively by them, without changing runtime behavior.

Scope Analysis (verified before planning)

A repo-wide search for each symbol confirmed the following:

Symbol	Only callers	Disposition
`append_recent_admin_dm_history`	called by `build_recent_admin_dm_history_messages` (also dead)	delete
`build_recent_admin_dm_history_messages`	no callers	delete
`AGENT_HISTORY_TURNS`	referenced only inside the dead `append_recent_admin_dm_history`	delete
`AGENT_HISTORY_QUERY_LIMIT`	defined but zero references repo-wide	delete
`append_simple_message`	many live callers throughout `agent.c`	keep
`nostr_handler_get_dm_history_json`	live via `tool_agent.c`	keep

No non-source references (Makefile, tests, docs) depend on these symbols.

Risks

Very low. Both functions carry __attribute__((unused)) so they are not linked into any runtime path. The only failure modes are accidental deletion of something still live, which the scope analysis above rules out.

Verification Strategy

Clean build with make — must succeed with no new warnings.
Existing tests under tests/ pass (python tests/run_tests.py or the project's usual invocation).
Manual sanity check: start an agent, send a DM, confirm normal reply — no regression, since the deleted code was never on the hot path.

Mermaid: before/after

flowchart LR
    subgraph Before
        A1[agent.c dead funcs] -. unused attr .- B1[compiler silences warning]
        A1 --> C1[reader assumes auto DM history replay]
        C1 --> D1[confusion about session behavior]
    end
    subgraph After
        A2[agent.c clean] --> E2[skill-driven history only]
        E2 --> F2[intent matches code]
    end

Follow-on (out of scope for this change)

The documentation in docs/CONTEXT.md already describes the skill-driven assembly model correctly, but neither that file nor docs/SKILLS.md explicitly states "DM history is not automatic — it only appears when a skill references it." A brief clarifying paragraph could be added in a separate change.

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Remove Dead DM-History Helper Functions in agent.c