feat: schema merging across endpoint observations (GLA-47)

[paoloanzn]

Summary

• Adds schema-merger.js module that merges per-request schemas into a single enriched schema when the same endpoint is observed multiple times
• Header merging: unions all header fields across observations, unifies value types into union strings (e.g. string | integer), preserves isAuth if any observation flagged it, marks headers absent from some observations as optional, tracks seenCount
• Body merging: uses schema-inferrer.unifySchemas for recursive structural merge — conflicting types become mixed with variants, newly seen fields marked optional, array item schemas unified
• Observation tracking: each endpoint in the export now includes observationCount and mergedSchema instead of just requestCount, giving downstream consumers confidence proportional to sample size

Test plan

• Capture multiple requests to the same endpoint (e.g. GET /api/users/123 and GET /api/users/456) — verify they group into one endpoint with observationCount: 2
• Verify mergedSchema in exported JSON contains unified field types across observations
• Test with responses where a field is null in one observation and string in another — merged schema should show mixed type with ["null", "string"] variants
• Test with responses where one observation has an extra field — merged schema should mark it optional: true
• Test with requests where some have an Authorization header and some don't — merged header schema should have isAuth: true and optional: true
• Export snapshot JSON and verify each endpoint entry has observationCount and mergedSchema

🤖 Generated with Claude Code

[linear]

GLA-47 Schema merging

When the same endpoint is observed again, merge the new schema into the existing one rather than overwriting.

• Union types when a field type conflicts across observations (e.g. null + string → string | null)
• Add newly seen fields as optional
• Confidence increases with each observation (track how many times each endpoint was seen)

Done when multiple observations of the same endpoint produce a single enriched schema.