Remove or reactivate deprecated CompactionHook (dead code still wired into lifecycle)

## Problem

`src/platform_agent/foundation/hooks/compaction_hook.py` contains a detailed 9-section compaction prompt template, but the `on_before_invocation` method **only logs a warning and does nothing**. The hook is marked as "retained for backward compatibility" yet it is still registered in the agent's hook chain during construction.

This creates several issues:
1. **False sense of security** — users may believe conversation compaction is active
2. **Dead code in the critical path** — the hook runs on every invocation but does nothing useful
3. **Confusion for contributors** — the 9-section prompt template suggests active functionality

## Proposed Solution

**Option A (Preferred): Remove from default hook chain**
- Remove `CompactionHook` from the default hooks loaded in `FoundationAgent`
- Keep the module as a reference implementation with clear documentation
- Add a note in ARCHITECTURE.md explaining that compaction is now handled by [mechanism X]

**Option B: Reactivate**
- If Strands SDK doesn't handle compaction natively, re-enable the `on_before_invocation` logic
- The existing 9-section format (system context, skill state, task description, decisions, file modifications, user messages, recent actions, blockers, next steps) is well-designed and worth preserving

## Context

The 9-section structured compaction format includes an "IRON RULE" to preserve verbatim user input, which is a good practice. If reactivated, the compaction threshold constants should be configurable via the harness:

- `_DEFAULT_TOKEN_THRESHOLD = 80000`
- `MAX_COMPACTION_TOKENS = 20000`
- `SYSTEM_PROMPT_TOKEN_RESERVE = 15000`

## Acceptance Criteria

- [ ] CompactionHook is either removed from the default hook chain or reactivated
- [ ] If removed: ARCHITECTURE.md documents what handles compaction instead
- [ ] If reactivated: thresholds are configurable via `DomainHarness`
- [ ] No dead code remains in the active hook chain

Start with src/platform_agent/foundation/hooks/compaction_hook.py and the FoundationAgent construction path to trace how CompactionHook enters the default hook chain. Read ARCHITECTURE.md and the DomainHarness context before choosing removal or reactivation; done means the hook is no longer dead in the active chain and the relevant documentation or configurable thresholds are addressed.

Written by the indexing model from the issue text.