Add user-defined translation rules (--rules)

[stackedsax]

What

Implements option 1 — the declarative translation-rules file — plus a small
README fix that missed the merge window.

--rules (user-defined translation rules)

Adjust a conversion without patching BAMMM. A rules file rewrites the SPLAT IR
between parse and emit, so one rule set works for any source/target pair:

bammm convert --from slurm --to pbs --rules my-rules.yaml job.sh

apiVersion: bammm.io/rules/v1alpha1
rules:
  - when: { equals: { spec.schedule.qos: debug } }
    set:  { spec.schedule.queue: debug-queue }
    warn: routed debug QoS to debug-queue
  - when: { to: pbs, has: spec.extensions.htcondor.requirements }
    remove: [ spec.extensions.htcondor.requirements ]
    warn: dropped HTCondor requirements (no PBS equivalent)

• internal/rules — when matches on from/to/has/equals; actions are
  default, set, rename, remove, with an optional warn. Full SPLAT
  document paths (spec.schedule.queue); evaluated over a generic map so
  remove genuinely drops fields.
• convert.ConvertWithRules threads the ruleset through and returns warnings
  (printed to stderr); Convert stays a thin nil-ruleset wrapper.
• Works for single and bulk (--input-dir) conversions.
• Worked example in examples/rules/; reference in docs/translation-rules.md
  (promoted from design → implemented).

CEL expressions and array-index paths are noted as future work.

Also: rescued README fix

Drops the internal (Partner)/(Community) tags from the scheduler table (they
track who validates each scheduler, not anything user-facing). This was
approved for #28 but pushed just after it merged, so it's cherry-picked here to
actually reach main.

Testing

• internal/rules unit tests: match gating, set/default/rename/remove, warn,
  nil-ruleset no-op.
• CLI tests: --rules routes a value end-to-end (slurm→pbs → #PBS -q debug-queue + stderr warning) and errors on a missing file.
• Full suite / vet / gofmt clean.

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