Fixes 5 critical issues in task-aware self-adaptive context pruning:

1. CRIT-1: Remove dead TaskAwareMig enum variant (was routed to TaskAware,
   never called MIG scoring). Fallback via FromStr with warning for backward compat.

2. CRIT-2: Add MIG eviction ordering test to verify lowest-MIG blocks evicted first.

3. CRIT-3: Enforce prune_protect_tokens in scored/MIG pruning paths
   (was missing). Extract common helper prune_protection_boundary().

4. CRIT-4: Document tokio::spawn limitation for goal extraction
   (not cancelled on shutdown). TODO(I3) for separate PR.

5. CRIT-5: Cap goal string length at 500 chars to prevent memory explosion.

Serde fix: Hand-written Deserialize impl routes through FromStr,
enabling graceful fallback for config files with removed task_aware_mig variant.

Tests: +31 (6065 → 6096 total)
- 3 serde round-trip tests
- MIG eviction ordering test
- prune_protect_tokens enforcement tests (scored/MIG paths)
- CLI and init wizard tests

Backward compatibility: Users with strategy="task_aware_mig" in config
files now receive a warning and fallback to reactive pruning.

CHANGELOG updated. Docs updated (book/src/changelog.md).

Main branch added Subgoal/SubgoalMig pruning strategies (#2022).
Our branch removed TaskAwareMig variant (dead code) and added hand-written Deserialize impl for graceful fallback.

This merge reapplies our CRIT-1 fixes (TaskAwareMig removal + task_aware_mig fallback)
while keeping the new Subgoal variants from main.

Changes:
- Removed TaskAwareMig variant reference (pruning_strategy was dead code)
- Added hand-written Deserialize impl routing through FromStr
- Added task_aware_mig → Reactive fallback with warning in FromStr
- Added 3 serde round-trip tests for backward compat
- Updated init.rs wizard to remove task_aware_mig option
- Updated summarization.rs match statement to remove TaskAwareMig arm

All tests pass, build clean.

…undary

The prune_protection_boundary() helper is only used in feature-gated code
(prune_tool_outputs_scored and prune_tool_outputs_mig). When building with
features that don't include context-compression (server, chat, ide), the
function was unreachable and caused a compiler error.

Fixes Bundle Check failures for server, chat, ide bundles on PR #2066.