docs(plans): specify the default-swarm-size project-config setting

[mxriverlynn]

What

Adds the feature specification for a new .han/config.md frontmatter setting, default-swarm-size (small | medium | large | dynamic, absent = dynamic), which sets a standing default band for the eight sizing-aware skills.

Planning artifacts (spec, decision log, team findings) live in docs/plans/config-default-swarm-size/.

Key behaviors specified

• A configured small/medium/large forces the band exactly like an explicit size argument; explicit user input still wins under the existing config precedence chain.
• dynamic (or no setting) keeps today's auto-classification, and dynamic is also accepted as an explicit per-run size to override a configured band for one run.
• An unrecognized size argument (typo) falls through to the config instead of silently bypassing it.
• Unusable values degrade with the existing one-line note; a bad config never fails a run.
• Doc surfaces in scope: config-rule.md (canonical + vendored copies), docs/configuration.md, docs/sizing.md, plus an ADR recording the reversal of the "sizing is overridable, not configurable" principle.

Review

Reviewed at size Small by han-core:junior-developer and han-core:edge-case-explorer; 13 findings recorded and resolved in artifacts/team-findings.md. One open item: extend the manual test plan (does not block implementation).