fix: keep per-wakeup content out of proactive agent system prompts

[shentry]

Closes #8473

Problem

Both proactive wakeup paths assembled req.system_prompt like this before calling build_main_agent:

req.system_prompt += "...previous conversation history:\n---\n{dump}\n---\n"   # changes every wakeup
req.system_prompt += WOKE_SYSTEM_PROMPT.format(cron_job=cron_job_str)          # changes every wakeup

build_main_agent then appends the persona, skills and tool declarations to that same string. So the whole static tail sits behind content that differs on every single run, the prefix cache can never match, and the cron runs in the issue report cached_tokens=None.

The issue reports the cron path. The background-task-result path in astr_agent_tool_exec.py had the identical construction and the identical problem, so this PR fixes both — fixing only one would have left the same bug in place one file over.

Change

Split the two wake prompts into a static system half and a dynamic user half, and move the per-run content into the user turn:

	before	after
static wake rules	system prompt	system prompt (unchanged)
cron job / background task payload	system prompt	user prompt
conversation history dump	system prompt	user prompt

The static rules deliberately stay in the system prompt, so the instructions keep their system role and "Proceed according to your system instructions" in the user turn stays accurate. Only the parts that actually vary move.

Result: for a given session the system prompt is byte-identical across wakeups, so persona + tools + rules form a stable cacheable prefix, and everything that varies rides in the last message where it was never cacheable anyway.

Tests

Added TestWokeMainAgentPromptCaching to tests/unit/test_cron_manager.py, which drives _woke_main_agent and captures the ProviderRequest handed to build_main_agent:

• conversation history reaches req.prompt, not req.system_prompt
• cron job payload reaches req.prompt, not req.system_prompt
• two wakeups differing in both history and payload produce an identical system_prompt (and different prompt)
• the static wake rules are still in the system prompt
• no history section is emitted for a fresh conversation

The first three fail against the previous implementation and pass with this change.

• pytest tests/unit/ -> 734 passed
• ruff format --check / ruff check (0.15.22) on the four touched files -> clean

Note on the full suite: pytest tests/ has 3 pre-existing failures in tests/test_dashboard.py (test_t2i_*), and ruff format --check flags 7 pre-existing test files. I verified both are present on a clean master checkout without my changes and are unrelated to this PR.

Follow-up not done here

The cron system prompt still starts with the wake rules, so its prefix differs from a normal chat turn's and the two paths cache separately. Sharing one prefix would mean letting build_main_agent append the wake rules after the persona/tool blocks, which is a bigger refactor than this fix needs. Happy to do it in a follow-up if you want it.

Summary by Sourcery

Ensure proactive agent wakeups keep dynamic per-run data out of the system prompt so provider prefix caching can be reused across runs.

Enhancements:

• Refine cron wakeup and background task wakeup prompt construction to split static system instructions from dynamic user content, improving prefix cache effectiveness.
• Centralize reusable user prompt templates for cron job context, background task context, and conversation history in astr_main_agent_resources for consistent behavior.

Tests:

• Add unit tests around cron wakeup requests to assert history and job payload are carried in the user prompt, the system prompt remains static across wakeups, and no history section is emitted for empty conversations.