chore(migration): 🧹 retire Empathy's Mantle source snapshot

[szmyty]

Parent: #79. Refs #92. Ownership context: #59 and #61.

Empathy still carries a historical Mantle implementation and local task/editor/workflow hookups. The useful missing capabilities are now reconciled into Mantle PR #40, at immutable destination e776b70eee24f8cbe5ad1760bf1355141cb23c4c. The Mantle owner PR is merged; this cleanup follows it.

This removes all 337 historical mantle/ files and three separately approved Mantle-owned staging files (340 source files total). It also removes the duplicate workflow and local Task/editor integrations. No replacement source checkout is introduced. No wrong-owner implementation was found within the snapshot; the existing Realm intake remains separate.

migration/mantle-removal.tsv records exact source modes/blobs and verified destination blobs for every snapshot file: 17 equivalent, 94 reconciled, 222 retained-current, four superseded. The staged-file gate passed for the three exact path/blob approvals before deletion. The tracked staging count decreases by exactly three, from 9,586 to 9,583, with zero files still assigned to Mantle.

Generated evidence is refreshed from canonical inputs. TASKS.md also reconciles an already-defined Identity task. The CSV removes two stale image rows for files already absent and updates three existing image classifications to current generator output; those image files are untouched. Historical audits remain intact.

Validation:

• 24 focused extraction/gitignore/staging/task tests pass.
• Frozen-tree root suite: 184 tests, 17 skips, one existing commitlint setup-node SHA assertion failure.
• EgoLint suite: 31 tests, one existing MegaLinter catalog SHA mismatch.
• Native PR fast-profile lint passes on the final head, with the snapshot publisher skipped. Canonical Ruff, Mypy, Secretlint, task catalog and staging ledger checks also pass.
• Three low-severity Bandit findings exactly match the base source; none introduced.
• Native automation reproduces the existing setup-node SHA assertion failure. OSV blocks on 208 high/critical findings, matching the checked-in base snapshot; dependency manifests/locks are unchanged.
• Mantle native Linux/macOS suites, static checks, install lifecycle smoke, and Identity verification pass: run 39095909596.

Canonical holistic validation completed on the exact frozen review head 3b094dd6749fa8fd00c4e637600c4ee624d35a90:

• Manual MegaLinter run 39124626317, attempt 1, event workflow_dispatch, canonical .mega-linter.yml, whole-codebase validation.
• Artifact megalinter-39124626317-1, ID 12135314176, SHA-256 5bdacf2f1b78025100a913aa14f08650a2278e7ab97c66f002be942a16995522; downloaded bytes verified against GitHub's artifact digest.
• Snapshot publisher skipped on the feature branch. No report snapshot publication was triggered.
• Detailed raw reports reviewed rather than normalized totals. All 101 linter IDs/statuses match the checked-in base: 69 success, 9 warning, 23 error. No diagnostic count increased; no introduced gating finding was identified.
• Mypy: identical 36 errors in four unchanged files. Bandit: identical 10 low-severity findings in unchanged calls. Secretlint: identical staged documentation example. No new type/security diagnostic.
• Lychee: 175 current source/URL/location errors, all already in the detailed baseline; zero introduced. Reported baseline count is 183 (184 detailed entries), with nine detailed entries disappearing. All links added by this migration returned 200; no surviving link targets a removed Mantle file.
• Other gating-log differences contain removed findings, source totals, timings, or config metadata. Warning-only diagnostics were reviewed; reduced copy/paste, Python, and prose totals reflect the removed snapshot.
• The overall holistic job is red for inherited repository lint debt; its status is not represented as a clean full-codebase result.

Mantle PR #40 is merged as d1b2a4d77b823df4aa215d4e1a7b4dd9fad9c5ed. Its merge tree exactly matches the tested owner tree. The maintainer authorized these coordinated merges in this work session.

See the migration record for source/deletion evidence and deferred scope. Mantle ADR backfill follows consolidation; Flow shortcuts and release integration remain queued for the agreed Flow/Realm milestones.