feat(feed): publish ordinary Content source

[kachofugetsu09]

Summary

• physically remove the MCP lifespan FeedPoller, manual poll tool, and proactive-only fetch/ack tools
• make an ordinary Core Timer plus content.source.v1 the single Feed poll owner
• move the shared domain backend to feed_runtime so MCP query/manage and Content source reuse one library
• freeze complete payload and ranking data for each event_id + content_hash revision
• settle only the exact revision so an old receipt cannot acknowledge newer content
• rotate MCP and source diagnostic logs at 5 MiB with three retained generations

History and ownership

• non-empty Feed exports and Content settlement remain fully traceable in their single authoritative stores
• empty polls do not submit Content or manufacture delivery history
• explicit unsubscribe remains the named user data-management path for deleting that source history
• diagnostic logs are bounded; authoritative Feed, Content, and Session facts are not rotated

Validation

• 38 tests passed against the exact Core composition
• real Manager + stdio MCP + Content + Timer fixtures passed
• candidate fixture proves private MCP handshake with zero Timer and zero formal DB writes
• reload fixture proves the old lifespan poll owner stops before the new Timer is scheduled
• Pyright: 0 errors and 0 warnings
• compileall, exact Plugin API v3 contract, and git diff --check passed
• exact feature head d3b9c84 received independent Terra xhigh APPROVE
• exact CI-boundary fix head 4ea1e2a received independent Terra xhigh APPROVE

The only test warning is the existing FastMCP/Pydantic lifespan forward-reference warning. The follow-up test-only commit excludes CI dependency checkouts from plugin fixture copying; it does not change product behavior or test assertions.

Recovery

Pre-change bundle: /mnt/data/coding/backups/feed-content-v3-20260823/pre-implementation-91f645ab.bundle
Pre-CI-fix bundle: /mnt/data/coding/backups/feed-content-v3-20260823/pre-ci-fix-d3b9c84/feed-content-v3-d3b9c84.bundle

This PR does not deploy or change the formal hua-home workspace.