feat: Agentic Trading session type (engine implementation, default-disabled)

[jordanrburger]

What this is

The implementation of the Agentic Trading subsystem in the engine — the follow-up to the design-only PR #171. This makes Agentic Trading a real, distributable Scout feature: any install can enable it, and it's generated by scoutctl bootstrap like every other session type.

⚠️ Optional, default-disabled, real-money. Places real brokerage orders via the robinhood-trading MCP. Ships off and does nothing until explicitly enabled. Not financial advice. See the design spec.

How it integrates (engine-native, not a vault copy)

The vault build used a dedicated launchd plist; the engine version instead uses the standard schedule dispatcher — a type: trading slot fires through the existing 5-minute tick. No new plist.

Layer	Change
schedule.py	SlotType.TRADING + SlotPriority.TRADING = 25; _PRIORITY_BY_TYPE entry
bootstrap.py	_assemble() gains a TRADING brain (sources phases/core + phases/trading, mode: [trading]); added to both stage-5 loops; run-trading.sh → _CAT1B_RUNNERS; trading.sh → _CAT1_TEMPLATES; trading-config.py → _CAT1_FILES_FROM_PLUGIN; config + 3 ledgers → _INSTALL_ONLY_TEMPLATES (never overwritten)
phases/trading/trading-loop.md	the 6-step brain (reconcile → risk gate → manage → enter → execute → log) + hard guardrails + headless degrade-to-PENDING-APPROVAL
phases/core/git-setup.md	opted into trading mode so the brain inherits the shared git/timezone/cost conventions
templates/	run-trading.sh.tmpl (guards on the master switch before any spend), scripts/trading.sh.tmpl (on/off/status), scripts/trading-config.py, config.yaml.tmpl (enabled: false + account placeholder), watchlist/decision-log/nav-history scaffolds
defaults/schedule.yaml	commented-out ET-pinned trading-market-open / trading-pre-close slots (opt-in)
CHANGELOG.md	[Unreleased] entry (manifests left for the release-time bump)

Two gates keep it safe

1. Master switch — config.yaml: enabled ships false; run-trading.sh exits before any spend when off, and the brain re-confirms in-session.
2. Opt-in schedule — the trading slots ship commented out; a fired slot is a harmless no-op until both the slot is uncommented and the config is enabled.

Verification

• End-to-end install into a temp vault renders all 9 artifacts; TRADING.md assembles with the loop + git-setup conventions and no briefing-phase leakage.
• ruff check ✓ · ruff format --check ✓ · mypy ✓ (69 files) · versioning check ✓ (4 manifests in sync) · shellcheck -S error ✓ on the templates.
• 882 tests pass (added test_install_seeds_agentic_trading_subsystem_disabled; extended the assembled-brains test to cover TRADING).

Deliberately out of scope (follow-ups)

• No connectors.yaml entry for Robinhood (the brain manages its own MCP availability) — avoids touching connector-health vocabulary.
• Manifest version bump is left to the normal release flow (this PR only adds to [Unreleased]).

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