Document the fishing minigame and milestone system in README

[dmccoystephenson]

Summary

• No open issues or PRs existed at triage time, and a full source sweep found no untested code, schema drift, or %d-on-float formatting bugs — the repo is in a clean, well-covered state (351 tests passing).
• This cycle is a Stage A documentation accuracy sweep (per the dev-loop skill's fallback work mode): the README's Features section documents selling fish, housing, and investment properties, but never described the fishing minigame itself (reaction timing, rod-level window, time-of-day and weather yield modifiers, weighted species rarity) or the milestone/achievement system (src/achievements/achievements.py) — both fully implemented and tested, but entirely absent from player-facing docs.
• Added two new Features subsections, Fishing and Milestones, matching the doc's existing tone and em-dash style. Every claim was verified against src/location/docks.py, src/fish/fish.py, and src/achievements/achievements.py before writing (e.g. the 10-energy-per-hour cost and 10-energy minimum to start match fish()'s hasEnergy/spendEnergy calls; the "announced once, even across reloads" claim matches getNewlyEarned's use of the persisted stats.earnedMilestones).
• PLANNING.md's FISH LOADING BAIT PRICE BUG goal note was investigated but left untouched — no corresponding bug was found in priceForBait load/save/display code (schema, PlayerJsonReaderWriter, and shop display all use %.2f correctly), and the note isn't specific enough to act on without guessing; left as-is rather than fabricating a fix under a docs-only cycle.

Test plan

• python3 -m compileall -q src
• Full pytest suite (headless SDL drivers): 351 passed, 0 failed
• Docs-only change — no code paths modified, so no regression risk

No tracking issue — gap found during this cycle's documentation sweep (Phase 2, Stage A).

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — only README.md is modified; the two new subsections are the sole change, no unrelated formatting/rename/comment churn.
• Tests-new: N/A — docs-only change, no new public methods/functions to cover.
• Tests-fix: N/A — not a bug fix.
• Sibling structure: PASS — the Fishing and Milestones sections use the same ### Heading + single-paragraph pattern and em-dash style as every other Features subsection (verified against Home Ownership/Investment Properties immediately above).
• Sibling renames: N/A — no identifiers renamed.
• Docs: PASS — this PR is the doc fix. Full Phase 7 table checked: schemas/*.json unaffected (no persisted fields touched), PLANNING.md investigated (FISH LOADING BAIT PRICE BUG note left as-is — no corresponding bug found in priceForBait load/save/display path, and the note is too vague to act on without guessing), version.txt not bumped (docs-only, not release-worthy).
• Issue resolution: N/A — no open issue existed for this gap; PR body states "No tracking issue — gap found during this cycle's documentation sweep."
• CI: PASS — gh pr checks 117 green (test workflow passed).

Repo-specific rubric (all N/A — no source files changed in this PR):

• Schema-sync: N/A
• Money-format: N/A
• Deterministic-tests: N/A
• Headless-pygame: N/A
• camelCase: N/A

Every factual claim in the new copy was traced to source before writing: energy cost/minimum from Docks.fish()'s hasEnergy/spendEnergy(10) calls, reaction-quality tiers and rod-level window from Docks.fish()/REACTION_BASE_WINDOW/ROD_WINDOW_STEP, time-of-day/weather modifiers from Docks.getTimeOfDayModifier/getWeatherModifier, species rarity from fish.FISH_TYPES, and the milestone "announced once, even across reloads" claim from achievements.getNewlyEarned's use of the persisted stats.earnedMilestones.

Summary: docs-only PR closing a real gap (two fully-implemented, tested gameplay systems — the fishing minigame and milestones — were undocumented in README's Features section). No code paths touched, so no regression risk; CI green.