Add Lexicon skill owning director prompt drift and size

[TheGreatAxios]

First-party slash-only skill: diffs same-name director prompts against the agents repo at a pinned commit, reports assembled prompt sizes via directorPromptSizeTable, and files drift issues through linear-issue-workflow conventions. No director surface, no Skywalker routing changes, ponytail untouched.

Fixes CL-7652

[linear-code]

CL-7652

[TheGreatAxios]

Audit: lexicon skill direction is good (drift + size + filed-issue reporting, explicit not-a-director / not-a-router guards). Two issues:

1. SKILL.md Step 1 pins a hardcoded checkout default /Users/thegreataxios/abklabs/agents — a personal machine path that is wrong on every other machine. Make it portable: probe candidate locations, accept a LEXICON_AGENTS_CHECKOUT-style env var, and otherwise require the operator path (fail closed, no silent default).
2. Staleness with Report first-party skill names on skill_used, bucket the rest as custom #959: this PR adds a first-party skill but Report first-party skill names on skill_used, bucket the rest as custom #959's closed FIRST_PARTY_SKILL_NAMES allowlist (and the TELEMETRY.md list) has no lexicon entry, so /lexicon usage would report as custom once Report first-party skill names on skill_used, bucket the rest as custom #959 lands. Add lexicon to FIRST_PARTY_SKILL_NAMES + TELEMETRY.md on this branch (merge order with Report first-party skill names on skill_used, bucket the rest as custom #959 will decide which side rebases, but one of them must carry it).

[TheGreatAxios]

Audit (fleet builder): two findings, one fixed here, one sequenced after #959.

1. Fixed (pushed): hardcoded operator checkout default. Step 1 resolved the agents checkout to /Users/thegreataxios/abklabs/agents — a machine-specific path that breaks every other operator. Now portable: probe ../agents beside the repo, then $AGENTS_CHECKOUT when set, else ask the operator; test pins no /Users/… path in the skill.

2. Sequenced, not skipped: lexicon belongs on the telemetry skill-name allowlist, but FIRST_PARTY_SKILL_NAMES does not exist yet — it is introduced by unmerged PR Report first-party skill names on skill_used, bucket the rest as custom #959, and neither main nor this branch has it (skill_used here still carries no name). Adding the entry belongs to a rebase-after-merge follow-up once Report first-party skill names on skill_used, bucket the rest as custom #959 lands; flagging here so it is not lost.