feat: add cycling-training Claude skill (MCP companion)

Modular cycling-training skill: periodization logic, workout design, and
research-backed protocols that companion an Intervals.icu MCP server (the
server owns all data + computed metrics; the skill owns knowledge + reasoning).

Structure:
- SKILL.md router + README (companion-to-MCP architecture, install steps)
- references/: durability, mtb-xco-demands, strength-for-cyclists,
  vo2max-intervals, periodization, data-confounds, citations
- assets/: base + build plan templates, durability field-test protocol

Every quantitative claim carries an inline citation; citations.md is the
single source of truth with PMID/DOI, each verified against PubMed on
2026-07-20. Notable honesty flags baked in:
- VO2 rep-length: running (Fleckenstein 2025) vs cycling (Ronnestad) conflict
  documented rather than asserted; cycling evidence favors short intervals.
- 'Yu et al. 2025' (frequency > distribution) could not be found; not cited.
- Under-researched doses (durability, taper TSB, CTL ramp) flagged 'track response'.
- HR-confound doc forbids naive decoupling<5% / RHR+5 rules.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TqrhBhC3GEcKyaw8RTk8G6

assets/plan-template-base.md

@@ -0,0 +1,62 @@
+# Plan template — base mesocycle (backward-mapped from an A-race)
+
+A fill-in skeleton for a **base** block. This is a *knowledge template*, not a calculator — pull
+every number (current CTL/ATL/TSB, FTP, MAP, TSS) from the Intervals.icu MCP server and reason with
+the logic in `references/periodization.md`.
+
+## Step 0 — anchor to the A-race (backward mapping)
+
+```
+A-race date:            __________
+Weeks until A-race:     __________
+Race demands:           (road/TT? MTB/XCO? marathon? — see references/mtb-xco-demands.md)
+Target race-week TSB:   +5 to +15   (see periodization.md → taper)
+Current CTL / ATL / TSB: ____ / ____ / ____   (from MCP get_athlete_summary)
+```
+Work **backwards**: peak → build → base. Base's job is to raise sustainable CTL and aerobic
+durability so the build block has something to sharpen.
+
+## Step 1 — mesocycle rhythm
+
+| Athlete profile | Loading rhythm |
+|---|---|
+| Younger / high recoverability | **3:1** (3 build weeks : 1 recovery week) |
+| **Masters / higher life stress** | **2:1** (2 build : 1 recovery) — favors recovery |
+
+Pick one and hold it. Recovery weeks are ~40–60% of the preceding load week's TSS.
+
+## Step 2 — weekly shape (base phase)
+
+Base is **mostly low intensity with a small hard dose kept alive.** Distribution is a principle,
+not a recipe (Seiler — see `references/periodization.md`): let volume, training age, and life
+stress set the exact split.
+
+```
+Weekly TSS target:        ____  (ramp CTL ~3–7 pts/week; see periodization.md → CTL ramp)
+Long endurance ride:      1 × ____ h   (durability driver — see references/durability.md)
+Endurance / Z2 rides:     __ × ____ h
+Aerobic-support intensity: 1 × threshold OR sub-threshold session (keep top-end from decaying)
+Strength:                 2 × /week heavy (see strength-for-cyclists.md; from-zero ramp if new)
+Recovery / off:           as needed
+```
+
+## Step 3 — build in durability early
+
+- Progressively lengthen the long ride; late in base, place **fatigued-state efforts late** in the
+  long ride (hard work after accumulated kJ) — see `references/durability.md`.
+- Fuel these to spare glycogen; the point is durability, not depletion.
+- Schedule the durability field test (`field-test-durability.md`) at the **start and end** of base
+  to measure the block.
+
+## Step 4 — strength through base
+
+- Base is the ideal time to build heavy-strength capacity: 2×/week, heavy compound lifts, 3–6 reps.
+- Sequence strength-first or on separate days to mute interference (`references/strength-for-cyclists.md`).
+- Plan to **preserve** (not abandon) strength into build/in-season — 1×/week maintenance.
+
+## Checkpoints (pull from MCP, judge with the skill)
+
+- [ ] CTL ramp within target (not runaway) — `get_athlete_summary`
+- [ ] Recovery weeks actually recovered — wellness trend, `references/data-confounds.md`
+- [ ] Durability % improving across the block — `field-test-durability.md`
+- [ ] Strength load progressing — logged sessions