Update merge mode schema and types with backwards-compatible transform

[acreeger]

Summary

Replace provider-specific merge mode enum values with generic ones and add a schema-level transform for backwards compatibility.

Context

Part of #842 — decoupling merge strategy from VCS provider. The mergeBehavior.mode setting currently uses provider-specific values (github-pr, github-draft-pr, bitbucket-pr) which create a combinatorial explosion as providers are added. This issue establishes the new canonical values that all other child issues consume.

Base branch: This work builds on top of the BitBucket integration branch (PR #609), which has bitbucket-pr already in the enum.

Acceptance Criteria

• mergeBehavior.mode accepts only local, pr, or draft-pr as canonical output values
• Old values (github-pr, github-draft-pr, bitbucket-pr) are accepted at parse time and automatically mapped to their generic equivalents via schema-level transform
• The transform applies to both schema definitions (the one with defaults and the one without)
• Schema .describe() text references the new generic mode names
• Telemetry event type for merge_behavior reflects the new canonical values
• Existing settings files on disk with old values continue to work without manual edits — the transform handles them transparently at load time

Shared Contracts

Produces:

• mergeBehavior.mode canonical output values: 'local' | 'pr' | 'draft-pr'
• Schema accepts old values and transforms: github-pr → pr, github-draft-pr → draft-pr, bitbucket-pr → pr
• LoomFinishedProperties.merge_behavior type: 'local' | 'pr' | 'draft-pr'

Note on telemetry continuity: After this change, analytics will receive 'pr' and 'draft-pr' instead of 'github-pr' and 'github-draft-pr'. Dashboards or queries filtering on the old values will need updating.

Scope Boundaries

• Does NOT update consumers of the mode value (other child issues handle that)
• Does NOT add a file-rewriting migration — the schema transform handles backwards compatibility at runtime
• Does NOT update code comments in files that aren't otherwise modified

Must-Haves

• substantive: src/lib/SettingsManager.ts — both Zod schemas define mode with canonical values ['local', 'pr', 'draft-pr'] and a transform accepting old values
• substantive: src/types/telemetry.ts — merge_behavior type is 'local' | 'pr' | 'draft-pr'