feat(FN-1553): reduce memory bloat with selective save behavior

- Update project memory instructions to encourage selective writes instead of unconditional appends
- Instruct agents to consolidate existing entries rather than add duplicates
- Add guidance to skip memory updates when no durable learnings were discovered
- Clarify what qualifies as durable learnings vs task-specific trivia
- Update user-facing memory documentation to reflect new selective behavior
- Add test coverage for selective memory write instructions

.changeset/reduce-memory-bloat.md

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+---
+"@gsxdsm/fusion": patch
+---
+
+Reduce project memory bloat and tighten memory save heuristics for selective curation.
+
+- Trim `.fusion/memory.md` from 64KB to 10.6KB (83% reduction) by removing task-specific FN-entries and keeping only durable, reusable learnings
+- Update `buildExecutionMemoryInstructions()` to require selective memory writes:
+  - Agents should skip memory updates when nothing durable was learned
+  - Agents should avoid task-specific trivia (logs, changelog entries, transient failures)
+  - Agents can consolidate/edit existing entries rather than always appending
+  - Only genuinely reusable insights qualify (patterns, conventions, pitfalls, constraints)
+- Update README.md with "What does NOT go in memory" section and selective save behavior
+- Update tests to assert new selective-save semantics