5-day challenge: resume triggers + Day 1 marker handoff from start-here

[SinparFP]

Summary

• five-day-challenge had no triggers for a returning owner ("let's keep going", "continue the challenge", "start day 2/3/4/5", "resume the challenge", "pick up where we left off"), so a mid-challenge session could fail to route into the skill at all.
• start-here never flipped the profile marker's challenge field off its template default of not-started, so a completed onboarding run looked like a fresh Day 1 to five-day-challenge and got re-run instead of resuming at Day 2.

What changed

• Added resume-phrase triggers to skills/five-day-challenge/SKILL.md.
• skills/start-here/SKILL.md Step 9 now sets challenge=day1 on a completed cold-start run (not if the run aborted before any win landed) — Day 1 of the challenge is start-here, so both entry paths converge on the same marker.
• Added a short clarifying note in five-day-challenge's Start section that a challenge=day1 marker may have come from a standalone start-here run, so it resumes at Day 2 rather than repeating Day 1.

Test plan

• Sonnet dogfood: fresh owner runs "start the 5 day challenge" → confirm marker lands on challenge=day1 after Day 1.
• Sonnet dogfood: owner runs /start-here standalone (never says "challenge"), then later says "continue the challenge" → confirm it resumes at Day 2 without re-onboarding.
• Confirm no other skill reads/writes the challenge= marker in a conflicting way (checked via grep — only these two files touch it).

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